You can read and analyze the .xls files using the free spreadsheet software included in open office

You can use either the data in .xls or .csv format (depending on your software). They contain the same data.

You can open and visualize .kml files in the free Google Earth. The files with the prefix "T" contain measured temperature departures from the traverse mean temperature (uncorrected), the files with the prefix "C" contain measured carbon-dioxide mixing ratios.